1. Field of the Invention
An engine control system, which performs learning control for adjusting an air-fuel ratio of a mixture of air and fuel, typically detects an air-fuel ratio of the mixture in order to calculate a correction value to bring the actual air-fuel ratio into line with a desired ratio. Such an adjustment or correction is accomplished by the use of the correction value in a feedback control. The control system is also typically used for adjusting the correction value so as to bring a basic fuel injection rate up or down to an appropriate rate. Because the physical properties of fuel injectors and fuel supply systems generally deteriorate due to aging, deviations of the actual air-fuel ratio from an ideal or desired value are produced. The learning control of an air-fuel ratio is useful, in an engine control system, to bring an actual air-fuel ratio back to the ideal or desired air-fuel ratio. Such an engine control system is known from, for instance, Japanese Patent Publication No. 62 - 59220.
2. Description of Related Art
Engine control systems, such as that disclosed by the Japanese publication referred to, typically have an engine provided with a fuel vapor purge system for preventing fuel vapor, generated in a fuel tank, from escaping into the atmosphere. When the air-fuel ratio learning control is conducted in a system including an engine having a fuel vapor purge system, however, its difficult to bring a basic fuel injection value back to a desired value, due to fuel vapor purged into the intake system. This is because when a zone in which the air-fuel ratio learning control is effected overlaps with a zone in which the fuel vapor purge is effected, the learning control for modifying the air-fuel ratio is executed during the purge of fuel vapor.
In order to eliminate adverse effects, produced by purging fuel vapor, on the learning control for adjusting an air-fuel ratio, it is desirable to have the engine control system alternate periodically between the purge of fuel vapor and the learning control for adjusting the air-fuel ratio, so that the purge of fuel vapor is suspended during execution of the learning control, and to increase the frequency of execution of purging the fuel vapor. However, if purging of fuel vapor is conducted alternately with the learning control in an engine having what is known as a "hot-wire" type of air flow sensor for detecting a flow rate of intake air introduced into the engine, a zone of engine operating conditions in which it is hard to control the air-fuel ratio with high accuracy is present. In short, the hot-wire type air flow sensor typically has a detection error which is small in a range of low air flow rates, but which becomes large as the air flow rate becomes higher. Hence, in the range of high intake air flow rates, even if a rate of fuel supplied supposedly corresponds to a flow rate of intake air actually detected by the hot-wire type air flow sensor, deviations in the air-fuel ratio occur easily. It takes a long time until the basic rate of fuel injection is brought to an appropriate rate, due to the deviations, as long as the learning control for adjusting the air-fuel ratio and the fuel vapor purge are alternately executed.